This invention relates to an active matrix liquid crystal display and, in particular, to an active matrix liquid crystal display of an IPS (In-Plane Switching) type.
In a liquid crystal display, an electric field is applied to a liquid crystal layer. To apply the electric field, various driving systems are known in the art. One such system is a static driving system in which a constant voltage signal is applied for each individual pixel to activate liquid crystals corresponding thereto. However, if the liquid crystal display has a large display capacity, the static driving system requires an enormously large number of signal lines and is not practically applicable.
Instead, a multiplex driving system is applied to the liquid crystal display of such a large display capacity. In the multiplex driving system, a signal voltage is supplied through a common signal line in a time division fashion. The multiplex driving system is classified into several types. Among others, an active matrix type using an active element in each pixel is known for its excellent display quality. The active matrix type is further classified into two types according to directions of the electric field applied to the liquid crystal layer. Specifically, one type is to apply the electric field in a direction perpendicular to glass substrates between which the liquid crystal layer is interposed. The other type is to apply the electric field in a direction parallel to the glass substrates and is therefore called an in-plane switching type (hereafter abbreviated to an IPS type). Such an IPS type is disclosed, for example, in Japanese Patent Publication (JP-B) No. 21907/1988. The IPS type is particularly adapted to a large-scale monitor because of its wide viewing angle.
At first, description will be made about the structure of a conventional active matrix liquid crystal display of an IPS type.
The conventional liquid crystal display comprises a first substrate portion with thin film transistors (sometimes abbreviated to TFT) formed therein, a second substrate portion opposite to the first substrate portion, and a liquid crystal layer held between the first and the second substrate portions.
The first substrate portion comprises a first glass substrate. A common electrode layer is formed or patterned on an inner surface of the first glass substrate that faces the liquid crystal layer. An insulator film is formed over the inner surface of the first glass substrate and the common electrode layer. On the insulator film, a video signal line layer and a pixel electrode layer are formed in a predetermined pattern. A protective insulator film is formed over the insulator film, the video signal line layer, and the pixel electrode layer. Over the protective insulator film, a first orientation film is formed to cause orientation in the liquid crystal layer. The first orientation film is prepared by rubbing treatment in a predetermined rubbing direction.
On the other hand, the second substrate portion comprises a second glass substrate. A light-shielding opaque metal film is formed in a matrix pattern on an inner surface of the second glass substrate that faces the liquid crystal layer. A second orientation film is formed over the inner surface of the second glass substrate and the opaque metal film. The second orientation film serves to cause orientation in the liquid crystal layer and is prepared by rubbing treatment in the predetermined rubbing direction like the first orientation film. The opaque metal film formed in the matrix pattern serves to prevent leakage of light in the liquid crystal display and is often called a black matrix.
Liquid crystals are confined between the first and the second substrate portions to form the liquid crystal layer. Finally, a first polarization plate is attached to an outer surface of the first glass substrate with its transmission axis coincident with the predetermined rubbing direction. On the other hand, a second polarization plate is attached to an outer surface of the second glass substrate with its transmission axis perpendicular to that of the first polarization plate.
Next, description will be made as regards operation of the liquid crystal display having the above-mentioned structure.
The first substrate portion further comprises a scanning signal line layer and a plurality of thin film transistors including a semiconductor layer. The scanning signal line layer comprises a plurality of scanning signal lines extending in a first direction. On the other hand, the video signal line layer comprises a plurality of video signal lines extending in a second direction perpendicular to the first direction. The pixel electrode layer comprises a plurality of pixel electrodes connected to the thin film transistors in one-to-one correspondence. Each of the thin film transistors is connected to one of the scanning signal lines and one of the video signal lines. In response to an ON/OFF signal from the scanning signal line layer, each of the thin film transistors including the semiconductor layer is turned on and off. When a particular one of the thin film transistors is turned on, electric charges flow from a corresponding one of the video signal lines to a corresponding one of the pixel electrodes. In response, those liquid crystals in a corresponding part of the liquid crystal layer are activated. Even after the thin film transistor is turned off, the electric charges flowing into the corresponding pixel electrode are maintained to keep a certain electric potential. Therefore, the liquid crystals are kept activated. On the other hand, the common electrode layer is continuously applied with a predetermined d.c. voltage.
The above-mentioned conventional liquid crystal display is disadvantageous as described in the following.
By way of example, consideration will be made about the case where a white window on a middle tone background is displayed. In this event, nonuniformity in luminance is caused in the middle tone background. Specifically, local difference in luminance is caused to occur although the uniform luminance is intended throughout the middle tone background. Such nonuniformity in luminance occurs in the manner which will hereafter be described.
Due to capacitive coupling present between the video signal line layer and the opaque metal film, the opaque metal film is subjected to potential modulation following potential variation of a video signal. This results in disturbance of a horizontal electric field applied to the liquid crystal layer. Since the opaque metal film is formed in the matrix pattern, the potential modulation concentrically spreads around a modulation point. Because of presence of capacitive coupling between the opaque metal film and the pixel electrode layer, the pixel electrode layer is subjected to potential modulation in response to the spread of the potential modulation of the opaque metal film. This again results in disturbance of the horizontal electric field applied to the liquid crystal layer. Such disturbance of the transversal electric field causes the nonuniformity in luminance to occur.
In view of the above, Japanese Unexamined Patent Publication (JP-A) No. 230074/1995 discloses the use of a nonconductive resin film as a light-shielding film formed in the second substrate portion. In other words, the above-mentioned opaque metal film is replaced by the nonconductive resin film in order to avoid the occurrence of the capacitive coupling. However, in order to achieve a light-shielding effect equivalent to that obtained by the opaque metal film, the resin film must have a thickness on the order of ten times that of the opaque metal film. Such a thick resin film provides a large unevenness between the surface of the resin film and the surface of the second glass substrate without the resin film. This brings about disclination of the liquid crystals and resultant occurrence of an afterimage.